This invention relates to a method for preparing a shell or porous gel from glass particles and, more particularly, to such a method in which glass particles are reacted, non-uniformly corroded or partially dissolved to convert solid glass particles into a product composed of shells, concentric shells or a porous, homogeneous gel which can be used in various applications.
Hollow glass microspheres have been produced in large quantities since the 1930""s (C. D. Hendricks, xe2x80x9cGlass Spheresxe2x80x9d, in Ceramics and Glasses, Vol. 4 Engineered Materials Handbook, 418-422, ASM International, USA (1991); and Neusy U.S. Pat. No. 4,487,620). Traditionally hollow glass microspheres have been processed by mixing glass forming raw materials (i.e. soda, lime and silica) with an organic material such as urea. These materials are mixed into a slurry, which is subsequently dried into a cake and crushed into small ( less than 200 xcexcm) particles. The particles are then introduced into a high temperature flame where the glass forming materials melt, and the gas released from decomposition of the organic material forms hollow glass microspheres. Many refinements of this general technique have been used to make the majority of hollow microspheres which are now commercially available. The main disadvantage of this method is that flame temperatures exceeding 1200xc2x0 C. must be used to produce the hollow microspheres. The hollow microspheres made using this xe2x80x9cburn outxe2x80x9d method are primarily used as fillers in light weight polymers, high temperature insulation and as fillers in paints (C. D. Hendricks, supra; L. H. Smiley, xe2x80x9cHollow Microspheres: More Than Just Fillers, Mech. Eng. Feb. 27-30 (1986); and R. H. Wehrenberg, xe2x80x9cShedding Pounds in Plastics: Microspheres are Movingxe2x80x9d, Mech. Eng. Oct. 58-63 (1978)).
Hollow glass microspheres can also be produced using sol-gel processing (R. L. Downs et al., xe2x80x9cHollow Glass Microspheres by Sol-Gel Technologyxe2x80x9d in Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, Ed. L. Klein, 330-380, Noyes Publications, NJ, (1988); L. C. Klein, xe2x80x9cSol-Gel Processxe2x80x9d in Ceramics and Glasses, Vol. 4 Engineered Materials Handbook, 209-214, ASM International, USA (1991); and R. D. Shoup, xe2x80x9cSol-Gel Processesxe2x80x9d, in Ceramics and Glasses, Vol. 4 Engineered Materials Handbook, 445-452, ASM International, USA (1991)). A sol is first prepared by mixing metal alkoxides, such as tetraethylorthosilicate (TEOS), with water and ethanol. This sol is then heated and acid catalyzed to form a silicate gel. The gel is dried and crushed into particles which are spheroidized in a high temperature ( greater than 1200xc2x0 C.) flame. While in the flame, the gel particles melt and decomposing organic groups release gas to form hollow glass microspheres. This technique is used to prepare hollow glass microspheres with very high purity and uniform wall thicknesses. The sol gel processing of hollow glass microspheres is very costly due to the high cost of the metal alkoxides used as raw materials. Sol gel processing also requires high temperatures and multiple processing steps which must be closely monitored. Thus, this technique is only used for very specialized applications such as the formation of hollow microspheres which are impregnated with deuterium and tritium for use in laser fusion experiments (R. H. Wehrenberg, supra).
There is a continuing need for improved processes for preparing a shell or porous gel from glass particles, particularly for processes which can be carried out at low temperatures ( less than 100xc2x0 C.) and which can produce hollow, porous microspheres of many different chemical compositions.
Among the several objects of the invention may be noted the provision of a method for preparing from glass particles a product composed of a shell filled with colloidal particles or gel, concentric shells, or a porous, homogenous gel; the provision of such a method by which porous/hollow shells may be produced at or near room temperature; the provision of a method of the type described which advantageously produces a porous product which may be filled with various types of liquids and gases; the provision of such a method which allows hollow/porous products of many different chemical compositions to be prepared; and the provision of a method for administering a chemotherapeutic drug which permits porous microspheres formed by glass particle/solution reactions to be dissolved with a chelating agent. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly, the present invention in one aspect is directed to a method for preparing a shell or porous gel from glass particles which involves the steps of:
(a) forming particles of an alkali borate glass composition containing one or more cations which react with an aqueous solution containing an anion reactive with the cation to form an aqueous insoluble material having a solubility limit of less than about 0.01 wt. percent;
(b) immersing the glass composition particles in the aforementioned aqueous solution so that the particles react and form the insoluble material which is essentially the same size as the as-made particles with the alkali and borate dissolving from the glass particles; and
(c) allowing the chemical reaction to continue until the alkali and boron are substantially completely dissolved from the glass particles and the resulting product is composed of a shell filled with colloidal particles or gel, concentric shells or a porous, homogeneous gel, the product being nonradioactive and not adapted for neutron irradiation.
In another aspect, the invention is directed to a method for administering a chemotherapeutic drug which involves the three above-noted steps and the additional steps of adding a chemotherapeutic drug to the above-noted resulting product and administering the then resulting product to a mammal, and thereafter dissolving the insoluble material from this product in vivo through the administration of a chelating agent. Another additional step may involve heat treating the above-noted resulting product so as to improve its mechanical strength or to control its porosity/permeability for the purpose of controlling the rate at which a drug is released or the product may be filled with a drug.